The present invention relates to an optical disk apparatus using an optical disk.
In general, in an optical disk apparatus, a laser beam from a laser diode is focused by an optical system, and an optical disk is scanned with the focused beam. Thus, binary data recorded on the optical disk is read.
Normally, when the focused beam is radiated on the optical disk, if the entire spot of the focused beam is located outside the pits, the phase of all the reflected light is the same. Thus, no decrease occurs in the amount of reflected light due to optical interference. On the other hand, when a part of the spot of the focused beam is within the pit, a phase difference occurs between the reflected light from the inside of the pit and that from the outside of the pit. Consequently, both reflected light components interfere with each other and the amount of reflected light decreases. In general, the optical disk apparatus is so designed that the amount of reflected light becomes minimum when the center of the focused beam spot lands on the center of the pit. Specifically, when the entire of the focused beam spot is located outside the pit as shown in FIG. 22, the level of the reproduced signal is high. When a part of the focused beam spot begins to overlap the pit, the level of the reproduced signal begins to decrease. When the focused beam spot is located at the center of the pit, the reproduced signal level takes a lowest value.
The density in operation of the optical disk apparatus has increased every year. One of the techniques for achieving high density is a technique for reducing the diameter of the focused beam. This requires a decrease in wavelength of a laser and an increase in NA (Numerical Aperture) of an objective lens. With the reduction of the diameter of the laser beam, information can be reproduced from smaller pits.
Even with the high-density optical disk apparatus wherein the focused beam diameter is reduced, however, information needs to be reproduced from a low-density optical disk which has already been marketed. In this case, as is understood from the relationship between the focused beam and the pit when information is to be reproduced from the low-density optical disk by the high-density optical disk apparatus, if the focused beam is located at the center of the pit, most of the focused beam is located within the pit as shown in FIG. 23 and the phase of most reflected light becomes the same. At this time, a decrease in the amount of reflected light due to interference is small. Specifically, when the focused beam is located outside the pit, the reproduced signal level is high. When the focused beam begins to overlap the pit, the reproduced signal level decreases. When the focused beam is at the center of the pit, the reproduced signal level increases once again. This phenomenon in which the reproduced signal level increases at the center of the pit is referred to as "rebounding".
Because of the rebounding, the reproduced signal level increases at the center of the pit, too, as shown in FIG. 24. Where the reproduced waveform in this case is detected by a waveform slice method, erroneous detection will occur even if the threshold is set at any level. When information is reproduced from the low-density optical disk by the high-density optical disk apparatus, as described above, there is a problem in that an error occurs in the signal detection result due to the rebounding.
In such an optical disk apparatus, there is a case where a compatibility capable of reproducing information among various types of optical disks having different recording densities such as CD (CD-ROM, CD-R etc.), DVD RAM, high density DVD-ROM and high density DVD-RAM of the coming generation is required.
Since, however, both the optimum wavelength of a light source and the shape of a light beam spot on the optical disk vary from optical disk type to optical disk type, it is generally difficult to correctly reproduce information from the plural types of optical disks by an optical disk drive using an optical head having a single light source and a single objective lens. Moreover, it is unfavorable to combine a plurality of light sources and a plurality of objective lenses in order to allow information from being reproduced from the plural types of optical disks having different recording densities because the optical head is increased in size and costs.
To resolve the above problem, for example, Jpn. Pat. Appln. KOKAI publication No. 8-339572 proposes an optical disk drive whose optical head is provided with a single light source, a single objective lens, and an opening limitation element having a plurality of openings of different sizes for limiting an opening of the objective lens to allow information to be reproduced from a plurality types of optical disks having different recording densities. In this optical disk drive, the diameter of a light beam spot on each of the optical disks is varied by the opening limitation element in accordance with the size of a pit corresponding to the recording density of an optical disk thereby to reproduce information from the plurality of types of optical disks having different recording densities.
On the other hand, in the conventional optical disk apparatus, in order to enable information reproduction from a plurality of kinds of optical disks with different recording densities, the recording density (size of pits) of the optical disks is merely considered and the opening size of an opening limiting element is varied. Since the opening size of the opening limiting element is not set in this apparatus in consideration of the relationship among the recording density of the optical disk, the light source wavelength and the pit depth of the optical disk, good reproduction is not achieved in the case of information reproduction from an optical disk which does not meet the conditions for the light source wavelength and pit depth. For example, the reproduced signal intensity decreases, or asymmetry of reproduced signals increases.